Our long-range goal is to develop recombinant human hemoglobin (rHb) as the starting material for manufacturing all globin-based O2 delivery Pharmaceuticals. We have established the key requirements for a rHb-based blood substitute: (a) moderate O2 affinity and large O2 dissociation rate constants for efficient transport in capillaries;(b) significantly reduced rates of NO scavenging to prevent hypertensive side effects;(c) resistance to auto- and chemically-induced oxidation to inhibit oxidative stress;and (d) low rates of heme dissociation to increase shelf-life. We have used the mechanisms governing these properties to design second-generation extracellular rHb-based blood substitutes with more efficient O2 transport and little or no hypertensive side effect in pre-clinical animal studies. However, there is a clear need to engineer third-generation, non-vasoactive rHbs with increased resistance to denaturation, enhanced expression yields, and reduced production costs. We propose to solve these problems by: (1) optimizing O2 binding to and NO scavenging by human rHb without compromising globin stability and resistance to degradative reactions;(2) improving the stability and enhancing production of recombinant hemoglobin in E. coli by (a) rational and comparative mutagenesis to increase the resistance of the apoprotein to unfolding and (b) co- expression of the alpha and beta rHb chains with the newly discovered erythroid chaperone, alpha hemoglobin stabilizing protein (AHSP);and (3) facilitating rapid incorporation of exogenously added heme into newly synthesized globins by co-expression of rHb with the heme utilization genes (hug) from Plesiomonas shigelloides and related pathogens. PUBLIC HEALTH: We are developing recombinant human hemoglobin to replace donated blood as the source material for all protein-based oxygen carriers that are being developed for the treatment of severe blood loss and believe that this effort should be a national public health priority. An effective hemoglobin- based blood substitute will alleviate chronic shortages of whole blood, provide emergency treatment following military and civilian disasters, and allow the use of transfusions in developing countries lacking a safe blood banking system.